1. Field of the Invention
The present invention relates to a method of driving a solid-state imaging device for discharging optical charge stored in channel areas to outside the channel areas.
2. Description of the Related Art
Conventionally, in an image pickup apparatus such as a television camera using a CCD solid-state imaging device, it has been conceived to effect exposure control electronically by making use of an operating principle of a CCD. As disclosed in Japanese Patent Laid-Open No. 24764/1988, such a method of exposure control is effected by transferring and discharging optical charge stored in an imaging portion up until a point in a period of photoelectric conversion for each vertical scanning period and by storing optical charge obtained by effecting photoelectric conversion during the remaining period of photoelectric conversion. That is, the period of photoelectric conversion is extended or shortened in correspondence with a change in the timing of discharge of optical charge, and the timing of discharge of optical charge is set in correspondence with the level of an output signal of the CCD.
With such a method of driving a CCD solid-state imaging device, unnecessary optical charge in the imaging portion is discharged by being transferred in an opposite direction to a direction in which reading and transfer is effected. However, in the discharge of optical charge by being transferred in the opposite direction, a problem is encountered in that smear occurs during the transfer and discharge in the same way as during reading and transfer. For that reason, various methods of discharging optical devices in which the smear does not occur have been conceived.
For instance, in Japanese Patent Application No. 96712/1989 filed by the present applicant, there is proposed a solid-state imaging device of a horizontal-type overflow drain structure with the drain disposed in parallel with a storage and transfer channel of an imaging portion, wherein a potential barrier between the storage channel and an overflow drain is extinguished before the optical charge is discharged from the storage and transfer channel to the overflow drain.
FIG. 3 is a top plan view of an essential portion of a CCD solid-state imaging device adopting the above-described driving method, and FIG. 4 is a cross-sectional view taken along the line X--X' of FIG. 3. Here, an imaging portion of a frame transfer-type CCD of a cross-gate structure is shown.
A plurality of channel stops 2 are arranged on one surface of a p-type semiconductor substrate 1 in parallel with each other by means of localized oxidation of silicon (LOCOS), an overflow drain 3 being formed below each of the channel stops 2. An n-type channel area 4 is formed between adjacent ones of the channel stops 2 in a diffusion process. A plurality of lower-layer electrodes 5 are provided in parallel on the channel areas 4 in a direction perpendicular to the channel stop 2, and a plurality of upper-layer electrodes 6 are provided along the respective channel stops 2. A projecting portion for covering each gap between adjacent ones of the lower-layer electrodes 5 is formed on each of the upper-layer electrodes 6 in such a manner as to be offset with the adjacent ones of the upper-layer electrodes 6.
The electrodes 5, 6 are driven by pulses by means of four-phase transfer clocks .phi..sub.F1 -.phi..sub.F4, and the transfer clocks .phi..sub.F1, .phi..sub.F3 and the transfer clocks .phi..sub.F2, .phi..sub.F4 are alternately applied to the upper-layer electrodes 6 and the lower-layer electrodes 5, respectively. In addition, a potential control clock .phi..sub.OFD is applied to the overflow drain 3. When optical charge is stored in the channel areas 4 by setting the potential of the transfer clock .phi..sub.F1 to high level and the potential of the control clock .phi..sub.OFD to low level, the potential inside the substrate 1 is made shallow between the channel area 4 and the overflow drain 3, as shown in FIG. 5, thereby forming a potential barrier. Accordingly, optical charge occurring in light-receiving areas 7 flow to below the projecting portions of the upper-layer electrodes 6 along the gradient of the potential and are stored in the channel areas 4.
Meanwhile, when the optical charge inside the channel areas 4 is to be discharged, the potential barrier between the channel area 4 and the overflow drain 3 is extinguished by setting the potential of the transfer clock .phi..sub.F1 to low level and the potential of the control clock .phi..sub.OFD to high level contrary to the case of storage, as shown in FIG. 5. When the potential barrier disappears, the optical charge inside the channel areas 4 flows to the overflow drains 3 along the potential gradient. Accordingly, the optical charge inside the channel areas 4 is discharged.
In accordance with such a method of discharging optical charge, it is possible to discharge the overall optical charge in the imaging portion substantially simultaneously and within a very short period of time, so that the smear is suppressed substantially in contrast to the method of discharging optical charge through reverse transfer.
However, when the optical charge is allowed to flow from the channel areas 4 to the overflow drains 3, there are cases where unnecessary optical charge remains in the channel areas 4. This is attributable to the concentration of impurities contained in such as the channel areas 4 and the overflow drains 3 as well as variations during manufacture in the thickness of the LOCOS portions of the channel stops 2 and the like, which creates areas where the potential barrier between the channel area 4 and the overflow drain 3 is liable to disappear and areas where it is difficult to disappear. If the potential at each area is made uniform, optical charge remains in areas where the potential barrier is difficult to disappear. To prevent the optical charge from remaining in this manner, a very high potential difference that will cause the potential barrier between the channel area 4 and the overflow drain 3 to disappear sufficiently needs to be provided between the channel area 4 and the overflow drain 3 for all the areas. Accordingly, there arises the problem that the voltage of the driving pulses for driving the CCD becomes disadvantageously high.
In addition, in Japanese Patent Application No. 96713/1989 filed by the present applicant, there is proposed a solid-state imaging device of a vertical type overflow drain structure with channel areas formed separately in a diffusion area on a semiconductor substrate, by increasing the potential of the semiconductor substrate where overflow drains are formed, the potential barrier between the storage and transfer channel and the semiconductor substrate is extinguished so as to discharge optical charge from the storage and transfer channel to the semiconductor substrate side.
FIG. 8 is a top plan view of an essential portion of a CCD solid-state imaging device adopting the above-described driving method, and FIG. 9 is a cross-sectional view taken along the line X--X' of FIG. 8. Here, an imaging portion of a frame transfer type CCD is shown.
A p-well area 22 is formed on one surface of an n-type semiconductor substrate 21, a plurality of p.sup.+ -type channel stop areas 23 are arranged in the p-well area 22 in parallel with each other. An n-type diffusion area 24 is formed between adjacent ones of the channel stop areas 23, thus constituting an embedded-type storage and transfer channel area. Transfer electrodes 25a, 25b are formed on the diffusion area 24 via an insulating film 26 in a direction perpendicular to the channel stop area 23. These transfer electrodes 25a, 25b form a double-layered structure, and the upper-layer transfer electrode 25b has its width narrowed on the channel stop area 23 and is disposed by straddling adjacent ones of the lower-layer transfer electrodes 25a. These transfer electrodes 25a, 25b are driven by pulses by means of four-phase transfer clocks .phi..sub.F1 -.phi..sub.F4, which are sequentially applied to the respective transfer electrodes 25a, 25b.
Meanwhile, a potential control clock .phi.sub is applied to the semiconductor substrate 21, and the p-well area 22 is secured to the grounded potential via the channel stop area 23. FIG. 10 illustrates the state of potential at a line Y--Y' (in FIG. 9) at the time when the potential control clock .phi.sub is thus applied to the semiconductor substrate 21 and a specific potential is applied to the p-well area. At this time, the transfer electrode 25b is held at a level higher by a fixed value with respect to the grounding level, and the semiconductor substrate 21 is held at a low potential, thereby forming a potential barrier in the vicinity of the p-well area 22. Accordingly, optical charge e is stored in a potential well formed between this potential barrier and the potential barrier on the surface of the semiconductor substrate 21. The transfer of this optical charge e is effected by varying the potential of each transfer electrode 25a, 25b within the range in which the potential barrier in the vicinity of the p-well area 22 is capable of maintaining a sufficient height.
Here, if the potential of the transfer electrodes 25a, 25b is set below a fixed level and the potential of the semiconductor substrate 21 is made high, the potential at the surface of the semiconductor substrate 21 becomes shallow, as shown by the broken line in FIG. 10, with the result that the potential inside the diffusion area 24 becomes shallow. Hence, the potential barrier in the vicinity of the p-well area 22 disappears, and the all the optical charge e flows to the semiconductor substrate 21 side. Accordingly, the discharge of the optical charge e stored in the storage and transfer channel can be effected by lowering the potential of the transfer electrodes 25a, 25b.
In accordance with such a method of discharging optical charge, it is possible to discharge the overall optical charge in the imaging portion substantially simultaneously and within a very short period of time, so that the smear is suppressed substantially in contrast to the method of discharging optical charge through reverse transfer.
However, when the optical charge is allowed to flow from the diffusion areas 24 to the semiconductor substrate 21 side, there are cases where unnecessary optical charge remains in the diffusion areas 24, in the same way as the above-described horizontal overflow drain structure. This is attributable to the concentration of impurities contained in such as the diffusion areas 24 and the p-well areas 22 as well as variations during manufacture in the thickness of these two types of areas 24, 22 and the like, which creates areas where the potential barrier in the vicinity of the p-well area 22 is liable to disappear and areas where it is difficult to disappear. If the potential at each area is made uniform, optical charge remains in areas where the potential barrier is difficult to disappear. To prevent the optical charge from remaining, a very high potential difference that will cause the potential barrier in the vicinity of the p-well area 22 to disappear sufficiently needs to be provided between the diffusion area 24 and the semiconductor substrate 21 for all the areas. Accordingly, there arises the problem that the voltage of the driving pulses for driving the CCD becomes disadvantageously high.